Method of producing cathode active material, and method of producing lithium ion battery

ABSTRACT

A major object is to provide a method of producing a cathode active material having a high average discharge potential, and a high degree of stability at high potential. The method includes: a step of preparing a Na-doped precursor of making a sodium-containing transition metal oxide having the P2 structure belonging to a space group of P6 3 /mmc; and an ion exchange step of substituting lithium for at least part of sodium contained in the sodium-containing transition metal oxide by the ion exchange method, wherein in the ion exchange step, at least lithium iodide is used as a Li ion source.

FIELD

The present application relates to a method of producing cathode active material, and a method of producing lithium ion batteries.

BACKGROUND

A layered cathode active material belonging to a space group of P6₃mc and having the O2 structure has been conventionally researched. It has been found that a cathode active material having the O2 structure is obtained by synthesis of a Na-doped precursor represented by the composition of Na_(2/3)(Ni,Mn,Co)O₂ and having the P2 structure in advance, and an ion exchange of Na contained in this Na-doped precursor for Li.

The reason why an ion exchange is employed is that the O2 structure is a metastable structure, and the most stable structure in a temperature range of room temperature to calcination temperature is the O3 structure, which make it difficult to directly synthesize the O2 structure. In addition, there are few compositions of a Na-doped precursor itself from which the P2 structure is obtained. The ternary system of Ni, Mn and Co limits the composition to such a range that charge neutrality is kept or therearound, as Ni is divalent, Mn is tetravalent and Co is trivalent.

In such a background, JP 2010-092824 A discloses a method of manufacturing a cathode active material, the method including: adding a mixture of lithium nitrate and lithium chloride to a sodium-containing transition metal oxide of the P2 structure belonging to a space group of P6₃/mmc, to substitute lithium for sodium contained in the sodium-containing transition metal oxide by the ion exchange method. Paragraphs [0039] to [0041] in JP 2010-092824 A show that the manufactured cathode active material has the O2 structure.

SUMMARY Technical Problem

A layered cathode active material of the O2 structure has the advantage of a small degree of capacity decay even if subjected to the cycle utilizing a high potential of 4.8 V to 5.0 V. In contrast, average discharge potential of a layered cathode active material of the O2 structure is not sufficient, and energy density thereof cannot be said to be sufficiently high either.

A major object of the present application is to provide a method of producing a cathode active material having a high average discharge potential, and a high degree of stability at high potential.

Solution to Problem

To solve the foregoing problems, the inventor of the present application thought that not a single-phase of the O2 structure but a multi-phase thereof and another structure can lead to both stability at high potential and sufficiently high energy density.

In JP 2010-092824 A, sodium contained in the sodium-containing transition metal oxide of the P2 structure is exchanged for lithium by the ion exchange method, to manufacture a cathode active material of the O2 structure. Like this, the ion exchange method is a method that makes it possible to convert structure of a sodium-containing transition metal oxide. It is believed that structure to be formed by the ion exchange method depends on the used Li ion source.

The inventor of the present disclosure subjected a sodium-containing transition metal oxide to an ion exchange using various Li ion sources, and then found that the use of at least lithium iodide makes it possible to produce a cathode active material having a multi-phase of the O2 structure and the O3 structure. Lithium iodide has not been used in the ion exchange method since having a strong reducing ability, and thus leading to the problem of low crystallinity. The inventor of the present disclosure however found out that subjecting a composition that comparatively easily leads to high crystallinity to an ion exchange intentionally using lithium iodide makes it possible to deliberately disrupt the O2 structure but form the O3 structure, to produce a cathode active material having a multi-phase thereof. The cathode active material having a multi-phase of the O2 structure and the O3 structure led to both high potential and a high degree of stability.

Based on the findings, the present application will hereinafter disclose aspects to solve the foregoing problems.

That is, the present application discloses, as one aspect to solve the foregoing problems, a method of producing a cathode active material, the method comprising: a step of preparing a Na-doped precursor of making a sodium-containing transition metal oxide having the P2 structure belonging to a space group of P6₃/mmc; and an ion exchange step of substituting lithium for at least part of sodium contained in the sodium-containing transition metal oxide by the ion exchange method, wherein in the ion exchange step, at least lithium iodide is used as a Li ion source.

In the method, the sodium-containing transition metal oxide preferably has the composition represented by Na_(a)Ni_(x)Mn_(y)Co_(z)O₂ where 0.5≤a≤1, x+y+z=1, and 3<4x+2y+3z≤3.5. In the ion exchange step, a mixture of lithium iodide and lithium nitrate is preferably used as the Li ion source. In this case, the ion exchange step is preferably carried out at a temperature of at most 400° C.

The present application discloses a method of producing a lithium ion battery, the method comprising: a step of making a cathode active material layer containing the cathode active material produced by the method of producing a cathode active material; and a step of using the cathode active material layer, an anode active material layer containing an anode active material, and an electrolyte layer containing an electrolyte, to arrange the electrolyte layer between the cathode active material layer and the anode active material layer.

In the method of producing a lithium ion battery, the electrolyte layer is preferably a solid electrolyte layer containing a solid electrolyte.

The present application also discloses, as one aspect to solve the foregoing problems, a cathode active material having the composition represented by Li_(b)Na_(c)Mn_(p)Ni_(q)Co_(r)O₂ where 0<b+c≤1, p+q+r=1, and 3≤4p+2q+3r≤3.5, wherein the O2 structure and the O3 structure coexist in a layered direction in a single particle of the cathode active material.

The present application discloses a lithium ion battery comprising: a cathode active material layer containing the cathode active material; an anode active material layer containing an anode active material; and an electrolyte layer containing an electrolyte, the electrolyte layer arranged between the cathode active material layer and the anode active material layer.

Advantageous Effects

The present disclosure can provide: a cathode active material having a high average discharge potential, and a high degree of stability at high potential; and a method of producing this cathode active material. The present disclosure can also provide a lithium ion battery using the cathode active material produced by the method, and a method of producing this lithium ion battery.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view of a surface of a cathode active material particle where a multi-phase of the O2 structure and the O3 structure is formed;

FIG. 2 is a schematic cross-sectional view of a lithium ion battery 100;

FIG. 3 shows the results of X-ray diffraction measurement of cathode active materials prepared in Example 1 and Comparative Example 1;

FIG. 4 shows the results of observation of structures of the cathode active materials prepared in Example 1 and Comparative Example 1; and

FIG. 5 shows the results of a charge-discharge test done for lithium ion batteries prepared in Example 1 and Comparative Example 1.

DESCRIPTION OF EMBODIMENTS Method of Producing Cathode Active Material

A method of producing a cathode active material which is one aspect of the present disclosure includes: a step of preparing a Na-doped precursor of making a sodium-containing transition metal oxide having the P2 structure belonging to a space group of P6₃/mmc; and an ion exchange step of substituting lithium for at least part of sodium contained in the sodium-containing transition metal oxide by the ion exchange method, wherein in the ion exchange step, at least lithium iodide is used as a Li ion source. This makes it possible to produce a cathode active material having a multi-phase of the O2 structure and the O3 structure.

Here, advantages and disadvantages in the case of a single-phase of the O2 or O3 structure will be described. A layered cathode active material having the O2 structure has the advantage of a high degree of stability at high potential, but has the disadvantage of insufficient average discharge potential. A layered cathode active material having the O3 structure has the advantage of a higher average discharge potential than the O2 structure but has the disadvantage of a low degree of stability at high potential which leads to a large degree of capacity decay after each cycle.

The cathode active material produced by the foregoing producing method is a layered cathode active material having a multi-phase of the O2 structure and the O3 structure. A cathode active material having a multi-phase of the O2 structure and the O3 structure as described above can overcome the disadvantage the O2 structure and the O3 structure each has, and further can enjoy the advantage thereof. The present disclosure can thus provide a method of producing a cathode active material which can provide a cathode active material having a high average discharge potential, and a high degree of stability at high potential.

Hereinafter the method of producing a cathode active material will be further described.

Step of Preparing Na-Doped Precursor

In the step of preparing a Na-doped precursor, a sodium-containing transition metal oxide having the P2 structure belonging to a space group of P6₃/mmc which is a Na-doped precursor is prepared.

The way of making the sodium-containing transition metal oxide is not particularly limited, and the sodium-containing transition metal oxide can be made by any known method. For example, the sodium-containing transition metal oxide may be made as follows: first, a Mn source, a Ni source and a Co source (any one or two of the elements may be omitted as necessary) are mixed to have such a proportion as to have a desired composition, and precipitated using a base; then a Na source is added to the precipitated powder to have such a proportion as to have a desired composition, and calcined. Precalcination may be carried out before the calcination. This makes it possible to obtain the sodium-containing transition metal oxide, which is a Na-doped precursor.

Here, each of the Mn source, the Ni source, and the Co source can be a nitrate, a sulfate, a hydroxide, a carbonate, or the like of a corresponding metal element, which may be a hydrate thereof. A base used for the precipitation can be sodium carbonate or sodium hydroxide, which may be used in the form of an aqueous solution. Further, an ammonia solution or the like may be added for adjusting basicity. The Na source can be sodium carbonate, sodium oxide, sodium nitrate, sodium hydroxide, or the like. The calcination can be performed at a temperature of 700° C. to 1100° C. Na is not sufficiently doped at a temperature lower than 700° C. Not the P2 structure but the O3 structure is formed at a temperature higher than 1100° C. The temperature is preferably 800° C. to 1000° C. A temperature when the precalcination is performed is equal to or lower than the calcination temperature. For example, the precalcination is performed approximately at 600° C.

The specific structure of the sodium-containing transition metal oxide is, for example, as follows: the sodium-containing transition metal oxide has the P2 structure belonging to a space group of P6₃/mmc, and has the composition represented by Na_(a)Ni_(x)Mn_(y)Co_(z)O₂ where 0.5≤a≤1, x+y+z=1, and 3<4x+2y+3z≤3.5. Here, the Na doping is within a range such that the P2 structure is formed, and such that charge neutrality is kept. The Na doping of less than 0.5 may lead to failure in synthesis of the P2 structure. One or two of x, y and z may be 0 each. That is, the sodium-containing transition metal oxide does not necessarily include one or two of Ni, Mn and Co. The composition of the sodium-containing transition metal oxide can be confirmed by, for example, X-ray diffraction (XRD).

Ion Exchange Step

The ion exchange step is a step of substituting lithium for at least part of sodium contained in the sodium-containing transition metal oxide by the ion exchange method, and is carried out after the step of preparing a Na-doped precursor.

In the ion exchange step, lithium is substituted for at least part of sodium contained in the sodium-containing transition metal oxide, using the ion exchange method. Sodium exchanged for lithium is preferably as much as possible. Lithium is more preferably substituted for at least 99% of sodium contained in the sodium-containing transition metal oxide. At least 99% is set in view of the measurement limits of measurement equipment such as ICP (at most 1%). Thus, the state where lithium is substituted for at least 99% of sodium means the state where Na is not detected when the composition after the ion exchange is measured by means of ICP or the like.

In the ion exchange step, at least lithium iodide is used as a Li ion source, to substitute lithium for sodium contained in the sodium-containing transition metal oxide. Examples of a Li ion source include lithium iodide, mixtures of lithium iodide and any other lithium salt, and Li-containing aqueous solutions containing lithium iodide (such as lithium hydroxide aqueous solutions containing lithium iodide). A mixture of lithium iodide and any other lithium salt is preferably used. Mixing of any other lithium salt with lithium iodide can lower heating temperature when the ion exchange is performed in the way of using heating as described later since being able to lower the melting point. The other lithium salt here is not particularly limited as long as the melting point of the mixture can be lowered than the melting point of lithium iodide, but is preferably lithium nitrate. When the mixture of lithium iodide and the other lithium salt is used, the mixing ratio is not particularly limited but the molar ratio of the lithium iodide:other lithium salt is preferably 5:95 to 30:70, and more preferably 10:90 to 20:80.

Examples of the way of making an ion exchange reaction progress include the way of using heat, and the way of using difference in concentration. Either way may be used in the method of producing a cathode active material, but the way of using heat is preferably used.

The way of using heat is a way of adding the Li ion source to the sodium-containing transition metal oxide, and heating the resultant, to make the ion exchange reaction progress. The use of heat makes it possible to achieve a desired ion exchange reaction for a short time. Examples of the Li ion source include a pure solid phase of lithium iodide, and molten salt bases formed of a mixture of lithium iodide and any other lithium salt. The heating temperature is at least equal to or higher than the melting point of the Li ion source. When a mixture of lithium iodide and any other lithium salt (such as lithium nitrate) is used, the heating temperature is preferably at most 400° C. because the heating temperature of at least 400° C. entirely changes the O2 structure to the O3 structure, which is a stable phase, for approximately 1 hour. The heating temperature is preferably at most 350° C. The lower limit is preferably at least 100° C. in view of a time taken for the ion exchange.

The heating time is not particularly limited as long as the sodium-containing transition metal oxide can be changed to a layered cathode active material having a multi-phase of the O2 structure and the O3 structure. The heating time is within the range of 30 minutes to 10 hours, and among them, preferably 30 minutes to 2 hours.

As the ratio of the amounts of the sodium-containing transition metal oxide and the Li ion source to be used, the amount of the Li ion source is preferably set so that the Li amount contained in the Li ion source is 1.2 times to 15 times, preferably 3 times to 15 times, more preferably 5 times to 15 times, and further preferably 8 times to 12 times as much as the sodium-containing transition metal oxide in terms of molar ratio.

The way of using difference in concentration is a way of making the concentration of Li contained in a Li-containing solution sufficiently high, to make the ion exchange reaction progress. This way has the advantage of a wide selection of materials because not needing heating. The Li ion source can be a Li-containing aqueous solution containing lithium iodide. The concentration of Li contained in the Li-containing solution is not particularly limited, and is usually within the range of 1 mol % to 10 mol %, and among them, preferably within the range of 4 mol % to 6 mol %. The reaction time is different according to the sodium-containing transition metal oxide etc., and is usually within the range of 10 hours to 300 hours, and among them, preferably 100 hours to 150 hours. Both heat and deference in concentration may be used together, to make the ion exchange reaction progress.

The ratio of the used sodium-containing transition metal oxide and Li-containing solution is different according to the concentration of Li contained in the Li-containing solution etc. For example, the Li-containing solution is preferably used within the range of 0.5 L to 1 L, and more preferably used within the range of 0.7 L to 0.8 L, to 100 g of the sodium-containing transition metal oxide because the foregoing range can efficiently make the ion exchange reaction progress.

Other Steps

The method of producing a cathode active material may include a step of pulverizing a cathode active material after the ion exchange step because the cathode active material is adjusted to have a desired shape and particle diameter when applied to a lithium ion battery. The pulverization can be carried out by, for example, ball milling by means of a ball mill.

Mechanism of Presumption

The method of producing a cathode active material makes it possible to control the sodium-containing transition metal oxide having the P2 structure belonging to a space group of P6₃/mmc to have structure having a multi-phase of the O2 structure and the O3 structure, by the ion exchange step. The reason why such control is enabled is because lithium iodide is used as a Li ion source in the ion exchange step.

When the structure is changed by the ion exchange, it is believed that whether the P2 structure is changed to the O2 structure or the O3 structure depends on the ionic exchange rate of the Li ion source. When the ionic exchange rate is high, a nonequilibrium reaction progresses, to form a more stable O3 structure. When the ionic exchange rate is sufficiently low, the O2 structure having the same oxygen stacking structure as the P2 structure is formed.

In JP 2010-092824 A, a mixture of lithium chloride and lithium nitrate is used. Since the ionic exchange rate of lithium chloride is sufficiently low, only the O2 structure is formed. In contrast, since lithium iodide has a high reducing ability (in other words, force of inserting Li into a material) compared to lithium chloride, the ion exchange progresses at a higher rate than in the case of lithium chloride. It is thus considered that the use of lithium iodide makes it possible to form structure where the O2 structure and the O3 structure coexist.

As another technique, there is a method of subjecting a sodium-containing transition metal oxide to an ion exchange with lithium chloride, and thereafter adding lithium iodide thereto. The purpose thereof is to additionally insert Li into a lithium-containing transition metal oxide using lithium iodide after the end of the ion exchange using lithium chloride. In such a method, however, the ionic exchange rate itself is determined by lithium chloride, and thus does not bring about a multi-phase of the O2 structure and the O3 structure.

Cathode Active Material

The cathode active material produced by the method of producing a cathode active material will be described. The cathode active material is a layered cathode active material having a multi-phase of the O2 structure (crystal structure is P6₃mc) and the O3 structure (crystal structure is R-3m). The multi-phase of the O2 structure and the O3 structure is specifically such that the O2 structure and the O3 structure coexist in the layered direction (c-axis direction, [001]) in a single particle. Such structure can be identified by selected area diffraction patterns or high-resolution images obtained using a transmission electron microscope, or by images obtained using a scanning transmission electron microscope. FIG. 1 is a schematic view of a surface of a cathode active material particle where the multi-phase of the O2 structure and the O3 structure is formed.

As shown in FIG. 1 , the cathode active material produced by the method of producing a cathode active material of the present disclosure has a multi-phase where the O2 structure and the O3 structure are layered. The ratio of the O2 structure and the O3 structure can be calculated from the intensity ratio of the 002 peak of the O2 structure and the 003 peak of the O3 structure which are determined using X-ray diffraction. The ratio of the O2 structure and the O3 structure calculated from this intensity ratio is preferably within the range of 5:1 to 1:1. This leads to a cathode active material having a further high average discharge potential, and a high degree of stability at high potential. Since fluctuating according to the composition, the positions of the 002 peak of the O2 structure and the 003 peak of the O3 structure cannot be described accurately. The person skilled in the art however can identify the positions.

The cathode active material has the composition represented by Li_(b)Na_(c)Mn_(p)Ni_(q)Co_(r)O₂ where 0<b+c≤1, p+q+r=1, and 3≤4p+2q+3r≤3.5. c is preferably as small as possible, and more preferably at most 0.01. c is at most 0.01 when lithium is substituted for almost entire sodium in the sodium-containing transition metal oxide, which is a precursor. In this case, writing of Na may be omitted. The composition of the cathode active material excluding Li and Na basically corresponds to the composition of the sodium-containing transition metal oxide, which is a precursor. The composition of the cathode active material can be confirmed by, for example, X-ray diffraction (XRD).

The cathode active material is preferably in the form of a particle. The average particle diameter of the cathode active material is, for example, within the range of 1 nm to 100 μm, and among them, preferably within the range of 10 nm to 30 μm.

Method of Producing Lithium Ion Battery

A method of producing a lithium ion battery using the cathode active material produced by the method of producing a cathode active material of the present disclosure will be described.

The method of producing a lithium ion battery, which is one aspect of the present disclosure, includes: a step of making a cathode active material layer 10 containing the cathode active material produced by the method of producing a cathode active material (hereinafter may be referred to as “step S1”); and a step of using the cathode active material layer 10, an anode active material layer 30 containing an anode active material, and an electrolyte layer 20 containing an electrolyte, to arrange the electrolyte layer 20 between the cathode active material layer 10 and the anode active material layer 30 (hereinafter may be referred to as “step S2”). FIG. 2 shows a schematic cross-sectional view of a lithium ion battery 100 that is one example of a lithium ion battery produced by the method of producing a lithium ion battery of the present disclosure. Hereinafter description thereof will be given with reference to FIG. 2 .

The method of producing a lithium ion battery of the present disclosure makes it possible to produce a lithium ion battery having a high average discharge potential, and a high degree of stability at high potential since the produced lithium ion battery includes the cathode active material layer containing the cathode active material produced by the method of producing a cathode active material of the present disclosure.

Step S1

Step S1 is a step of making the cathode active material layer 10 containing the cathode active material produced by the method of producing a cathode active material of the present disclosure. The way of making the cathode active material layer 10 is not particularly limited. The cathode active material layer 10 can be made by any known way: that is, may be made in a wet condition or may be made in a dry condition. For example, one may disperse materials to constitute the cathode active material layer 10 in an organic solvent to make a slurry, apply the slurry onto a cathode current collector 40, and dry to form the cathode active material layer 10. Or, one may mix materials to constitute the cathode active material layer 10 in a dry condition, and carry out press molding to form the cathode active material layer 10.

Cathode Active Material Layer 10

The material constituting the cathode active material layer 10 include at least the cathode active material produced by the method of producing a cathode active material of the present disclosure. The material may optionally include a binder, a conductive material, and a solid electrolyte. The thickness of the cathode active material layer 10 is not particularly limited but suitably adjusted according to the structure of the battery, and is preferably within the range of 0.1 μm to 1 mm.

Cathode Active Material

The foregoing cathode active material is used for the cathode active material here. The content of the cathode active material in the cathode active material layer 10 is not particularly limited. For example, the cathode active material is preferably contained in an amount of 60 mass % to 99 mass %, which is more preferably 70 mass % to 95 mass %, when the total mass of the cathode active material layer 10 is 100 mass %.

Binder

The binder is not particularly limited as long as being chemically and electrically stable. Examples thereof include fluorine-based binding materials such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE), rubber-based binding materials such as styrene-butadiene rubber (SBR), olefinic binding materials such as polypropylene (PP) and polyethylene (PE), and cellulose-based binding materials such as carboxymethyl cellulose (CMC). The content of the binder in the cathode active material layer 10 is not particularly limited, but is preferably within the range of 1 mass % to 40 mass %.

Conductive Material

The conductive material is not particularly limited, and any known one as a conductive material for lithium ion batteries may be employed. Examples thereof include carbon materials. Carbon materials include acetylene black, Ketjenblack, VGCF (vapor-grown carbon fiber), and graphite. The content of the conductive material in the cathode active material layer 10 is not particularly limited, but is preferably within the range of 5 mass % to 40 mass %, and more preferably within the range of 10 mass % to 40 mass %.

Solid Electrolyte

The solid electrolyte is not particularly limited as long as having desired ionic conductivity. Examples thereof include oxide solid electrolytes and sulfide solid electrolytes. Examples of oxide solid electrolytes include lithium lanthanum zirconate, LiPON, Li_(1+X)Al_(X)Ge_(2−X)(PO₄)₃, Li—SiO-based glasses, and Li—Al—S—O-based glasses. Examples of sulfide solid electrolytes include Li₂S—P₂S₅, Li₂S—SiS₂, LiI—Li₂S—SiS₂, LiI—Si₂S—P₂S₅, Li₂S—P₂S₅—LiI—LiBr, LiI—Li₂S—P₂S₅, LiI—Li₂S—P₂O₅, LiI—Li₃PO₄—P₂S₅ and Li₂S—P₂S₅—GeS₂. The solid electrolyte may be amorphous, and may be crystalline. The solid electrolyte is preferably in the form of a particle. The average particle diameter of the solid electrolyte is, for example, within the range of 1 nm to 100 μm, and among them, preferably within the range of 10 nm to 30 μm. The content of the solid electrolyte in the cathode active material layer 10 is not particularly limited, but preferably within the range of 1 mass % to 40 mass %.

Step S2

Step S2 is a step of using the cathode active material layer 10, the anode active material layer 30 containing an anode active material, and the electrolyte layer 20 containing an electrolyte, to arrange the electrolyte layer 20 between the cathode active material layer 10 and the anode active material layer 30. While the way of arranging these layers is different according to the aspect of the electrolyte layer 20, these layers may be arranged by any known way. For example, when the electrolyte layer 20 is a liquid electrolyte layer, one may arrange a separator (such as a porous separator made from PP) between the cathode active material layer 10 and the anode active material layer 30, and fill a place where the separator is arranged with a liquid electrolyte, to make the electrolyte layer. Or, when the electrolyte layer 20 is a solid electrolyte layer, one may stack the layers so as to arrange the solid electrolyte layer between the cathode active material layer 10 and the anode active material layer 30, and apply a load to press them as necessary.

Electrolyte Layer 20

The electrolyte layer 20 is a layer arranged between the cathode active material layer 10 and the anode active material layer 30, and contains at least an electrolyte. The electrolyte layer 20 conducts ions between the cathode active material and an anode active material via the electrolyte contained therein. The aspect of the electrolyte layer 20 is not particularly limited. The electrolyte layer 20 may be a liquid electrolyte layer, a gel electrolyte layer, a solid electrolyte layer, or the like. Among them, the electrolyte layer 20 is preferably the solid electrolyte layer containing the solid electrolyte.

The liquid electrolyte layer is usually a layer formed by using a nonaqueous electrolyte solution. The nonaqueous electrolyte solution usually contains a supporting salt and a nonaqueous solvent. The supporting salt dissociates in the solvent, to generate a charge carrier (lithium ion). Examples of the supporting salt include fluorine-containing lithium salts such as LiPF₆ and LiBF₄. Examples of the nonaqueous solvent include nonfluorine or fluorine-containing carbonates. The carbonates include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), and monofluoroethylene carbonate (FEC), and linear carbonates such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and methyl 2,2,2-trifluoroethyl carbonate (MTFEC). For example, the nonaqueous electrolyte solution may contain one or two or more optional components as necessary, in addition to the foregoing supporting salt and nonaqueous solvent.

For example, the gel electrolyte layer can be obtained by addition of a polymer to a nonaqueous electrolyte solution to gelatinate the nonaqueous electrolyte solution. Specifically, a polymer such as polyethylene oxide (PEO), polyacrylonitrile (PAN) or polymethyl methacrylate (PMMA) can be added to the nonaqueous electrolyte solution to gelatinate the nonaqueous electrolyte solution.

The solid electrolyte layer is a layer formed by using a solid electrolyte. The solid electrolyte is not particularly limited as long as having Li ion conductivity. Examples thereof include oxide solid electrolytes and sulfide solid electrolytes. Examples of oxide solid electrolytes include lithium lanthanum zirconate, LiPON, Li_(1+X)Al_(X)Ge_(2−X)(PO₄)₃, Li—SiO-based glasses, and Li—Al—S—O-based glasses. Examples of sulfide solid electrolytes include Li₂S—P₂S₅, Li₂S—SiS₂, LiI—Li₂S—SiS₂, LiI—Si₂S—P₂S₅, Li₂S—P₂S₅—LiI—LiBr, LiI—Li₂S—P₂S₅, LiI—Li₂S—P₂O₅, LiI—Li₃PO₄—P₂S₅ and Li₂S—P₂S₅—GeS₂. The solid electrolyte may be amorphous, and may be crystalline. The solid electrolyte is preferably in the form of a particle. The average particle diameter of the solid electrolyte is, for example, within the range of 1 nm to 100 μm, and among them, preferably within the range of 10 nm to 30 μm.

The content of the solid electrolyte in the solid electrolyte layer is preferably at least 80.0 mass %, more preferably at least 90.0 mass %, further preferably at least 95.0 mass %, and further preferably at least 99.0 mass %, when the total mass of the solid electrolyte layer is defined as 100 mass %. The upper limit is not particularly limited. The solid electrolyte layer may be formed of the solid electrolyte only.

The solid electrolyte layer may contain a binder to bind solid electrolyte particles to each other in view of development of plasticity etc. The foregoing binder may be used as the binder here. In view of preventing low ionic conductivity of the solid electrolyte layer, the content of the binder is preferably at most 20 mass %, more preferably at most 10 mass %, further preferably at most 5 mass %, and further preferably at most 1 mass %, when the total mass of the solid electrolyte layer is defined as 100 mass %.

The thickness of the solid electrolyte layer is suitably adjusted according to the structure of the battery, and is usually, but is not particularly limited to, 0.1 μm to 1 mm.

Examples of the way of forming such a solid electrolyte layer may include pressure molding on a powder of materials of the solid electrolyte layer including the solid electrolyte, and other components as necessary, to form the solid electrolyte layer. As another way thereof, one may apply a slurry for the solid electrolyte layer which contains the binder onto a support, dry up the slurry for the solid electrolyte layer, and remove the support, to form the solid electrolyte layer.

Anode Active Material Layer 30

The anode active material layer 30 contains at least an anode active material, and may optionally contain a binder, a conductive material, and a solid electrolyte. The thickness of the anode active material layer 30 is not particularly limited but suitably adjusted according to the structure of the battery, and is preferably within the range of 0.1 μm to 1 mm.

Anode Active Material

Any known anode active material used for lithium ion batteries may be used as the anode active material. For example, a silicon-based active material such as Si, Si alloys, and silicon oxide; a carbon-based active material such as graphite and hard carbon; any oxide-based active material such as lithium titanate; lithium metal or a lithium alloy; or the like may be used. The content of the anode active material in the anode active material layer 30 is not particularly limited. For example, the anode material is preferably contained in an amount of at least 60 mass %, which is more preferably at least 70 mass %, and further preferably at least 80%, when the total mass of the anode active material layer 30 is defined as 100 mass %. The upper limit of the content of the anode material is not particularly limited, and the anode active material layer 30 may consist of the anode material. In view of the content of optional components, the anode active material layer 30 may contain in an amount of at most 99 mass % of the anode material, which may be at most 95 mass %

The same ones, which may be employed for the cathode active material layer 10, may be employed for a conductive material and a binder that may be contained in the anode active material layer 30. They are optional components, and the contents thereof are not particularly limited either. The configuration thereof may be the same as in the cathode active material layer 10.

The way of making the anode active material layer 30 is not particularly limited. The anode active material layer 30 can be easily made in a dry or wet condition as well as the cathode active material layer 10.

Other Steps

The method of producing a lithium ion battery may further include a step of arranging the cathode current collector 40 on a face of the cathode active material layer 10 which is on the opposite side where the electrolyte layer 20 is arranged (hereinafter may be referred to as “step S3”), and a step of arranging an anode current collector 50 on a face of the anode active material layer 30 which is on the opposite side where the electrolyte layer 20 is arranged (hereinafter may be referred to as “step S4”). Steps S3 and S4 may be carried out after step S2, or may be carried out before step S2. Step S3 may be carried out as being included in step S2 (step S4 may be done as being included in step S2) since one can make materials to constitute the cathode active material layer 10 (anode active material layer 30) a slurry, and apply the slurry to the cathode current collector 40 (anode current collector 50), to make the cathode active material layer 10 (anode active material layer 30).

Cathode Current Collector 40

Examples of the material of the cathode current collector 40 include SUS, aluminum, nickel, iron, titanium, and carbon. The cathode current collector 40 may be, for example, in the form of foil, in the form of mesh, or in a porous form. Stacking the cathode current collector 40 onto the cathode active material layer 10 makes it possible to easily make a cathode. The cathode current collector 40 may be omitted according to the materials contained in the cathode active material layer 10. In this case, the cathode active material layer 10 itself is the cathode.

Anode Current Collector 50

Examples of the material of the anode current collector 50 include SUS, aluminum, nickel, copper, and carbon. The anode current collector 50 may be, for example, in the form of foil, in the form of mesh, or in a porous form. Stacking the anode current collector 50 onto the anode active material layer 30 makes it possible to easily make an anode. The anode current collector 50 may be omitted according to the materials contained in the anode active material layer 30. In this case, the anode active material layer 30 itself is the anode.

A step of sealing a lithium ion battery into a case or the like may be further included. A general battery case may be used as the case without any particular limitations. Examples thereof include a battery case made of SUS.

Lithium Ion Battery

The lithium ion battery produced by the method of producing a lithium ion battery of the present disclosure is as described above: that is, as one example shown in FIG. 2 , is the lithium ion battery 100 including the cathode active material layer 10 containing the cathode active material of the present disclosure, the anode active material layer 30 containing the anode active material, and the electrolyte layer 20 containing the electrolyte, which is arranged between the cathode active material layer 10 and the anode active material layer 30. The electrolyte layer 20 is preferably the solid electrolyte layer containing the solid electrolyte.

As described above, in the lithium ion battery 100, the cathode current collector 40 may be arranged on a face of the cathode active material layer 10 which is on the opposite side where the electrolyte layer 20 is arranged, and the anode current collector 50 may be arranged on a face of the anode active material layer 30 which is on the opposite side where the electrolyte layer 20 is arranged. Further, the lithium ion battery 100 may be sealed into the case or the like.

The lithium ion battery 100 may be a primary battery, and may be a secondary battery. The lithium ion battery 100 is preferably a secondary battery in view of a more effective improvement in durability. This is because secondary batteries can be repeatedly charged and discharged, and are useful for, for example, automotive batteries. Secondary batteries include batteries used as a primary battery (used for the purpose of discharging once after charging). The lithium ion battery 100 may be, for example, in the form of a coin, a laminate, a cylinder, or a rectangle.

EXAMPLES Making Lithium Ion Battery Example 1 Synthesizing Na-doped Precursor

Mn(NO₃)₂.6H₂O (manufactured by FUJIFILM Wako Pure Chemical Corporation) and Ni(NO₃) (manufactured by FUJIFILM Wako Pure Chemical Corporation) as raw materials were dissolved in pure water so that the molar ratio of Mn and Ni was 2:1. A Na₂CO₃ solution of 12 wt % in concentration was prepared. These two solutions were titrated in a beaker at the same time. At that time, the titrimetric rate was controlled so that pH was at least 7.0 and lower than 7.1. After the titration, the mixed solution was stirred for 24 hours under the conditions of 50° C. and 300 rpm. The obtained reaction product was washed by pure water, and only a precipitated powder was separated by centrifugation. The obtained powder was dried under the conditions of 120° C. and 48 hours, and thereafter crushed in an agate mortar.

Na₂CO₃ (manufactured by Sigma-Aldrich) was added to the obtained powder to be mixed so that the composition ratio was Na_(0.8)Ni_(1/3)Mn_(2/3)O₂. The mixed powder was pressed at a load of 2 ton by cold isostatic pressing, to make a pellet. The obtained pellet was precalcined in the atmospheric air under the conditions of 600° C. and 6 hours, and thereafter calcined under the conditions of 900° C. and 24 hours, to synthesize a Na-doped precursor (Na_(0.8)Ni_(1/3)Mn_(2/3)O₂).

Ion Exchange Step

LiNO₃ (manufactured by NACALAI TESQUE, INC.) and LiI (manufactured by Sigma-Aldrich) were mixed so that the molar ratio was 88:12, and the mixed powder was weighed so that the amount of Li contained in the mixed powder was 10 times as much as that of the Na-doped precursor in terms of molar ratio. The Na-doped precursor and the mixed powder were mixed, and heated in the atmospheric air under the conditions of 280° C. and 1 hour, to be subjected to an ion exchange. After the ion exchange, water was added to dissolve the residual salts, and further a rinse is given to obtain a cathode active material (Li_(0.8)Ni_(1/3)Mn_(2/3)O₂) which was a material to be aimed. The obtained cathode active material was subjected to ball milling, to be a powder.

Making Lithium Ion Battery

Into a 125 mL n-methylpyrrolidone solution that was a solvent of dissolving 5 g of polyvinylidene fluoride (PVDF), which was a binding material, 85 g of the obtained cathode active material, and 10 g of carbon black, which was a conductive material, were introduced, to be kneaded until being uniformly mixed, to make a paste. This paste was applied to one face of a Al current collector having a thickness of 15 μm so that the weight thereof was 6 mg/cm², to have the paste having a thickness of 45 μm and density of 2.4 g/cm³, to obtain an electrode. At last, this electrode was cut out so as to have a diameter of 16 mm, to obtain a cathode.

Li foil was cut out so as to have a diameter of 19 mm, to obtain an anode.

Coin cell, CR2032 was made using the obtained cathode and anode. A porous separator made from PP was used as a separator. EC (ethylene carbonate) and DMC (dimethyl carbonate) were mixed so as to have the volume ratio of 3:7, and lithium hexafluorophosphate (LiPF₆) was dissolved in the mixture as a supporting salt so as to have a concentration of 1 mol/L, which was used as an electrolyte solution.

Comparative Example 1

A cathode active material (Li_(0.7)Ni_(1/3)Mn_(2/3)O₂) and a lithium ion battery were prepared in the same manner as in Example 1 except that a powder such that LiNO₃ and LiCl were mixed so as to have a weight ratio of 88:12 was used in the ion exchange step.

Evaluation X-ray Diffraction Measurement

X-ray diffraction measurement was taken on the cathode active materials synthesized in Example 1 and Comparative Example 1. The results are shown in FIG. 3 . As shown in FIG. 3 , while the sharp diffraction peak showing the O2 structure is obtained in Comparative Example 1, the diffraction peak is wide, and a plurality of diffraction peaks concurs in Example 1. This reflects that the crystallite size in Example 1 is smaller than Comparative Example 1.

Observation of Structure

Next, the structures of the particles of the cathode active materials synthesized in Example 1 and Comparative Example 1 were observed using a scanning transmission electron microscope. The results are shown in FIG. 4 . As shown in FIG. 4 , it was found that while being all the O2 structure in Comparative Example 1, the structure has a multi-phase where the O2 structure and the O3 structure coexist in the C-axis direction [001] in Example 1.

Charge-Discharge Test

Lastly, the lithium ion batteries made in Example 1 and Comparative Example 1 underwent a charge-discharge test. Specifically, one cycle of charging at 0.1 C to 4.8 V, and discharging at 0.1 C to 2.0 V was performed. The results are shown in FIG. 5 . In Example 1, the discharge capacity was 199.2 mAh/g, and the average discharge potential was 3.52 V. In Comparative Example 1, the discharge capacity was 192.8 mAh/g, and the average discharge potential was 3.28 V. As the foregoing, it was found that the discharge capacity was larger and the average discharge potential was much higher in Example 1 than those in Comparative Example 1. This seems to be because the cathode active material used in the lithium ion battery of Example 1 had a multi-phase of the O2 structure and the O3 structure.

REFERENCE SIGNS LIST

-   10 cathode active material layer -   20 electrolyte layer -   30 anode active material layer -   40 cathode current collector -   50 anode current collector -   100 lithium ion battery 

What is claimed is:
 1. A method of producing a cathode active material, the method comprising: a step of preparing a Na-doped precursor of making a sodium-containing transition metal oxide having a P2 structure belonging to a space group of P6₃/mme; and an ion exchange step of substituting lithium for at least part of sodium contained in the sodium-containing transition metal oxide by an ion exchange method, wherein in the ion exchange step, at least lithium iodide is used as a Li ion source, and O2 structure and O3 structure coexist in a layered direction in a single particle of the cathode active material by the ion exchange step.
 2. The method according to claim 1, wherein the sodium-containing transition metal oxide has the composition represented by Na_(a)Ni_(x)Mn_(y)Co_(z)O₂ where 0.5≤a≤1, x+y+z=1, and 3<4x+2y+3z≤3.5.
 3. The method according to claim 1, wherein in the ion exchange step, a mixture of lithium iodide and lithium nitrate is used as the Li ion source.
 4. The method according to claim 3, wherein the ion exchange step is carried out at a temperature of at most 400° C.
 5. A method of producing a lithium ion battery, the method comprising: a step of making a cathode active material layer containing the cathode active material produced by the method according to claim 1; and a step of using the cathode active material layer, an anode active material layer containing an anode active material, and an electrolyte layer containing an electrolyte, to arrange the electrolyte layer between the cathode active material layer and the anode active material layer.
 6. The method according to claim 5, wherein the electrolyte layer is a solid electrolyte layer containing a solid electrolyte.
 7. A cathode active material having the composition represented by Li_(b)Na_(c)Mn_(p)Ni_(q)Co_(r)O₂ where 0<b+c≤1, p+q+r=1, and 3≤4p+2q+3r≤3.5, wherein O2 structure and O3 structure coexist in a layered direction in a single particle of the cathode active material.
 8. A lithium ion battery comprising: a cathode active material layer containing the cathode active material according to claim 7; an anode active material layer containing an anode active material; and an electrolyte layer containing an electrolyte, the electrolyte layer arranged between the cathode active material layer and the anode active material layer. 